Magnetic resonance angiography (MRA) is the magnetic resonance imaging of the blood vessels in the body. In MRA, special pulse sequences are used by an MR scanner to cause flowing blood to appear very bright and stationary tissue to appear very dark. If arterial structures are being studied, additional pulses are applied to erase the signal in veins. Multiple thin slices are obtained at adjacent levels through the region of interest. In prior techniques, a computer then stacks these images and creates a three-dimensional image. The constructed image can be rotated 360 degrees so that the vessels can be studied in all projections.
MRA has become a primary method for the evaluation of vascular pathologies and increasingly for the purposes of surgical planning. Rapidly improving data acquisition methods have greatly improved image quality. Much less attention, however, has been focused on post-processing techniques for enhancing what is captured in the image. One such method, the maximum intensity projection (MIP), has become the standard for vascular visualization. Because of its simplicity and non-parametric basis as well as its high visual quality, the MIP is generally advocated when the angiogram is of high quality. However, limitations in image quality in MRA persist when it has been applied to more challenging conditions such as in the abdomen, extremities, the heart, and where the vascular tree is highly overlapped such as in the cerebral MRA. The limited image quality affects the accuracy of analysis of vessel shape for diagnosis of vascular disease and the accuracy of determination of vessel paths for surgical planning.
Specifically, there are several difficulties inherent in image analysis of the vasculature. One is the lack of definition in the vascular structure; within any finite resolution image, the distal extent of the vascular tree is indeterminate. Thus, the number and extent of detected vessels is dependent on acquisition and analysis methods. Another difficulty is that, even in moderately large vessels (by axial dimension), the vessel diameter will tend to be small relative to the image resolution. A third difficulty is that vascular shape and anatomy is generally quite complex and variable.
An objective of the invention, therefore, is to provide a method for delineating vessels in angiograms. Another objective of the invention is to provide such a method that works with vascular images of the type normally obtained from a magnetic resonance angiogram.